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MICROBIOLOGY STANDARD OPERATING PROCEDURE
URINE CULTURE
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Applies to:
Microbiology laboratory
Modified by:
1
Sep-2005
Date of revision:
Date for review:
Aim
To isolate, quantify and permit presumptive identification and differentiation of the major
microorganisms causing urinary tract infections (UTIs).
2
Principle
All urines undergo a dipstick test and/or microscopy to look for the presence of white blood cells,
red blood cells, nitrites and bacteria. All children under 3 years of age should have a microscopy
performed since dipsticks can be unreliable in this age group due to frequent voiding.
A known volume of urine is cultured in order to allow quantification of the number of organisms in
the original urine, although because of imprecisions in the method this is usually referred to as
‘semi-quantitative’ culture.
The UTI chromogenic medium (Oxoid Brilliance™ UTI Clarity™ agar) contains two specific
chromogenic substrates. The different dyes produced by the organisms leads to different urinary
isolates appearing as different coloured colonies after overnight incubation at 37ºC in air.
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URINE CULTURE
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3
Method
3.1 Specimen collection
Urine specimens may be collected in several ways:





Mid-stream specimen
Clean catch specimen
Bag specimen
Catheter specimen (either from an in-out catheter or an indwelling catheter)
Supra-pubic aspirate (needle directly into the bladder)
Wherever possible a specimen of urine should be collected aseptically directly into a sterile
universal container following cleaning of the perineal area.
3.2 Specimen transport and storage
Specimens should ideally be stored and transported in sealed plastic bags. Laboratory processing
should occur as soon as possible after specimen collection. Specimens should be refrigerated if
delays in processing over two hours are unavoidable.
3.3 Specimen processing
3.3.1 Reception
Log the specimen in the appropriate specimen book and assign a specimen number.
3.3.2 Pre-culture examination
Perform a urine dipstick and microscopy on all urine samples arriving in the microbiology
laboratory.
3.3.2.1 Microscopy using the Kova slide technique
Tip the closed urine pot over to mix carefully then use a capillary tube to place unspun urine into
one chamber of the Kova slide, leave the chamber on the bench for one minute for the cells to
settle.
Using a low power microscope objective (x40), count the RBC or WBC cell numbers in 36 small
grids, unless obviously >100 (i.e. >1 per small grid).Thirty six small grids is equivalent to four large
squares on the Kova slide: therefore, count cells in all of the small grids form the four corner large
squares.
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Multiply the average number of RBC or WBC cells per grid (total number cells / 36) by 90 to
determine the number of cells per microlitre (L).
Follow Table 1 for the cell numbers to report.
Table 1. Calculating the number of cells per microlitre using the Kova slide
Total cells
counted
Cells/L
Total cells
counted
Cells/L
1
1/36*90 = 3
13
33
2
5
14
35
3
8
15
38
4
10
16
40
5
13
17
43
6
15
18
45
7
18
19
48
8
20
20
50
9
23
25
63
10
25
30
75
11
28
40
100
12
30
>40
>100
3.3.3 Culture
If the urine microscopy is positive or one of the following points are true, culture the urine:





Sample screened by counting chamber has over 10 WBC/µL (104 WBC/mL; i.e. ≥5 cells
counted (in 36 small squares)
Follow up of patients on treatment
Urinary tract obstruction
Follow up after removal of indwelling catheter
Child of less than 3 years who has suspected urinary tract infection
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URINE CULTURE
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
Suspected melioidosis patient: culture on Ashdown’s medium in addition to standard
media
Describe the appearance of the urine (clear or cloudy) and colour: note this in the laboratory book
and on the computer.
Turn the urine pot over to mix it carefully and remove the top of the container.
Dip the end of a sterile 1L loop into the urine and remove it vertically making sure that there is no
urine up the loop (as this would mean that a greater volume was cultured).
Spread the entire volume over the surface of a Brilliance UTI Clarity agar plate by making a single
streak across the centre. Spread the inoculum evenly at right angles to the primary streak as
shown in Figure 1. If many samples are being processed use half a plate per sample.
Incubate the plate aerobically at 35-37°C for at 18-24 hours.
After inoculation, estimate the number of bacteria by counting the number of colonies on the
surface of the media. One colony = 1,000 cfu/mL (1x106 cfu/L).
Figure 1. Streaking the urine specimen onto a Brilliance UTI Clarity agar plate
4
Interpretation
Culture results are categorised on the basis of quantity (Table 2) and purity (Table 4) of growth.
Below a recognised threshold (105 cfu/mL) the likelihood is that the organisms grown are
contaminants, particularly if more than one type of organism is present. Above the threshold it is
more probable that a true urinary tract infection is occurring.
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Table 2. Relationship between colony count and quantity of bacteria in the urine specimen
Colony count
Number organisms per mL of urine
<10
<104 cfu/mL
10-100
104 – 105 cfu/mL
>100
>105 cfu/mL
If there is a pure growth of 10-100 or over 100 colonies, sub culture the isolate for identification and
antimicrobial susceptibility testing.
For cultures that contain two organisms, one in low numbers (<100 colonies) and the other over
100 colonies, then only sub-culture the predominant organism because the organism of lower
numbers is unlikely to be causing disease. If both are present at over 100 colonies, sub-culture
both organisms.
If more than two organisms are isolated, then do not sub-culture / identify any of them since this is
highly likely to be a contaminated specimen.
An exception is that all growth from a supra-pubic aspirate specimen must be fully identified and
have sensitivity tests performed.
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4.1 Minimum level of identification in the laboratory
Presumptive identification can be made from the chromogenic agar plate (Figure 2).
Figure 2. Identification of bacterial species from the Brilliance UTI Clarity agar plate
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Confirmatory testing should be done on all organisms, if necessary make purity plates on blood
agar first (Table 3).
Table 3. Confirmatory identification tests for urinary tract isolates
Organism
Identification methods
Staphylococci
Catalase
Coagulase / Staphaurex
DNase agar
Novobiocin disc if coagulase negative (S. saprophyticus is
resistant)
Streptococci
Catalase
Enterococcus spp.: group and sub onto bile-aesculin agar
Other streptococci: sub on blood agar (+optochin disc) and
identify using conventional tests (haemolysis, group etc.)
GNB
E. coli: confirm with indole only (inoculate MIL medium)
Other coliforms: ID using biochemistry short set +/- API 20E
Pseudomonas spp.: confirm as Pseudomonas aeruginosa with
oxidase and growth of green colonies on Columbia agar after
incubation at 42C. If not P. aeruginosa report as
Pseudomonas sp. (ID with API 20NE only if clinically indicated)
Other organisms (e.g. yeasts)
Only if felt to be clinically significant
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4.2 Antimicrobial susceptibility testing
All significant isolates should have antimicrobial susceptibilities determined according to SOP MIC001.
4.3 Reporting
4.4 Reporting of results from microscopy
1. Report the number of WBC and RBC per mL in urine using the counting chamber.
2. Comment on the presence of epithelial cells, bacteria, casts or yeasts (see Appendix 1)
following the following counts (using x40 objective and Kova slide):
 No cells
=
none seen
 <1 per field (not small grid)
=
+/-l
 1 – 10 per field
=
+
 11 – 25 per field
=
++
 >25 per field
=
+++
 Seen under other cells (cannot count) =
present
3. Report the presence of Trichomonas vaginalis.
4. Casts are solidified protein which are cylindrical in shape as they are formed by the
kidney tubules
5. If culture is not indicated report "Culture not done because white blood cell count was
below significant levels (104 WBC/mL)".
4.5 Reporting results from culture
Culture result
Report
No bacterial growth
No growth
Single organism
<104 CFU/ml
104 -105 CFU/ml
>105 CFU/ml
No significant growth
Growth of 104 -105 cfu/ml of organism, ?significance
Report antimicrobial sensitivities
Growth of >105 cfu/ml of organism
Report antimicrobial sensitivities
Two organisms
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Both <105 CFU/ml
One >105 CFU/ml
Both >105 CFU/ml
>2 organisms
5
No significant growth (please repeat if appropriate)
Mixed growth including >105cfu/ml of organism, ?significance
Report antimicrobial sensitivities of the one >10 5 cfu/ml only
Mixed growth of >105cfu/ml of organism1 and organism2, ?significance
Report antimicrobial sensitivities for both
Mixed growth of >2 organisms (please repeat if appropriate)
Quality assurance
Media and identification tests should be quality controlled according to the relevant SOP.
6
Limitations
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Label plates with the date of preparation and store the prepared medium at 2-8°C. The shelf life
after preparation is two weeks after which plates should be discarded.
Only use the media that has passed QC.
Organisms with atypical enzyme patterns may give anomalous results; e.g. white colonies may
occasionally prove to be E. coli on further examination.
7
References
1. NICE Guidelines. Urinary tract infection in children, diagnosis, treatment and long-term
management. Clinical Guideline, August 2007.
2. Health Protection Agency, UK SOP B41: Investigation of Urine (Issue 7.1; December
2012).
3. Oxoid information page for Brilliance™ UTI Clarity™ agar Code: CM1106.
4. Cheesbrough, M. District Laboratory Practice in Tropical Countries, Part 2. 2nd Edition
Update (2006). Cambridge University Press.
5. Kova slide manufacturer’s protocol.
6. Standard Operating Procedures from LOMWRU, SMRU and AHC.
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8
Synopsis / Bench aid
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9
Risk assessment
COSHH risk assessment - University of Oxford COSHH Assessment Form
Description of procedure
Culture of urine
Substances used
Variable, depending on organism cultured (may
include Gram stain reagents; 3% hydrogen
peroxide (catalase test); N,N,N',N'-tetramethyl-1,4phenylenediamine (oxidase test); sodium
deoxycholate (bile solubility test); bioMerieux API
reagents)
Frequency of SOP use
Daily
Could a less hazardous substance be
used instead?
No
Quantities of chemicals used
Small
Hazards identified
1. Autoclaved liquid
2. Potentially infectious material in sample
3. Potentially pathogenic bacteria
What measures have you taken to control risk?
1. Training in good laboratory practices (GLP)
2. Appropriate PPE (lab coat, gloves, eye protection)
3. Use of biosafety cabinet for reading of plates / follow-up of BSL-3 organisms (e.g. B.
pseudomallei)
Checks on control measures
Observation and supervision by senior staff
Is health surveillance required?
Training requirements:
No
GLP
Emergency procedures:
Waste disposal procedures:
1. Report all incidents to Safety Adviser
1. Sharps discarded into appropriate rigid
2. Use eyewash for splashes
containers for incineration
3. Clean up spills using 1% Virkon or
2. Infectious waste discarded into autoclave bags
chemical spill kit
or 1% Virkon solution prior to autoclaving and
subsequent incineration
3. Chemical waste disposed of according to
manufacturer’s instructions
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10 Appendix 1: Useful images for urine microscopy
Red cells, White cells, and bacteria
Red Blood Cell
White Blood Cell
Bacteria
Epithelial cells (indicate that the urine is not a clean catch)
White cell casts (found when there is inflammation of the kidney pelvis or tubules)
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Red cell casts (indicate haemorrhage into the renal tubules or glomerular bleeding; orange red
colour)
Hyaline casts (associated with damage to the glomerular filter membrane)
`
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Crystals
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